# Indexed performance idealized pressure curve in the cylinder of a steam engine. The area outlined in red is a measure of the internal work done

The indicated power (also inside power , internal power or indicated horsepower  (PSI) is given), the performance in a piston engine , which results from the to the piston , during a working cycle fluctuating acting gas pressure and the rotational speed is obtained. The indicated power differs from the output (effective) power because the machine has mechanical losses, for example due to friction and auxiliary drives such as oil and cooling water pumps. In internal combustion engines or piston steam engines , the internal power is slightly greater than the effective one, in compressors it is smaller than that supplied from the outside for the same reason.

In order to determine the indicated performance, the pressure curve in the cylinder is determined during a work cycle. In the slow-running steam engines of the 19th century, you could directly connect an indicator , a writing measuring device (a manometer with a pen as a pointer and a piece of paper moved by the piston), which shows the work as the area covered by the pen. Modern devices work with piezoelectric pressure sensors and electronic angle sensors , the signals of which are processed by microprocessors .

With the amount of energy determined in this way per work cycle and cylinder , the indicated power of a cylinder can be calculated: ${\ displaystyle W _ {\ mathrm {Zi}}}$ ${\ displaystyle P _ {\ mathrm {Zi}}}$ ${\ displaystyle P _ {\ mathrm {Zi}} = i \ cdot n \ cdot W _ {\ mathrm {Zi}}}$ With

• Number of work cycles per crankshaft revolution (1 for two-stroke engines , 0.5 for four-stroke engines )${\ displaystyle i}$ • rotational speed ${\ displaystyle n}$ • the indexed work of a work cycle is equal to the area from a pressure-volume diagram ( pV diagram ).${\ displaystyle W_ {Zi}}$ The quotient of the effective power per cylinder and the indicated power is the mechanical efficiency of the piston machine: ${\ displaystyle P _ {\ mathrm {Zeff}} = {\ tfrac {M} {Z}} \ cdot \ omega}$ ${\ displaystyle P _ {\ mathrm {Zi}}}$ ${\ displaystyle \ eta _ {\ mathrm {mech}}}$ ${\ displaystyle \ eta _ {\ mathrm {mech}} = {\ tfrac {P _ {\ mathrm {Zeff}}} {P _ {\ mathrm {Zi}}}}}$ The mechanical efficiency is always less than 1 and depends on the load.

In the past, the unit of measurement  PSi was often used when specifying indexed services to refer to the feature “indexed” in the size value - instead of, as is customary in modern times, in the size designation.